EP3249838A1 - Mode de transmission et sélection de débit pour un système de communication sans fil - Google Patents

Mode de transmission et sélection de débit pour un système de communication sans fil Download PDF

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Publication number
EP3249838A1
EP3249838A1 EP17179652.7A EP17179652A EP3249838A1 EP 3249838 A1 EP3249838 A1 EP 3249838A1 EP 17179652 A EP17179652 A EP 17179652A EP 3249838 A1 EP3249838 A1 EP 3249838A1
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European Patent Office
Prior art keywords
age
channel
station
snr
channel information
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EP17179652.7A
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German (de)
English (en)
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Santosh Abraham
Arnaud Meylan
Jay Rodney Walton
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Qualcomm Inc
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Qualcomm Inc
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0417—Feedback systems
    • H04B7/0421—Feedback systems utilizing implicit feedback, e.g. steered pilot signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
    • H04L1/0002—Systems modifying transmission characteristics according to link quality, e.g. power backoff by adapting the transmission rate
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/0001—Systems modifying transmission characteristics according to link quality, e.g. power backoff
    • H04L1/0023—Systems modifying transmission characteristics according to link quality, e.g. power backoff characterised by the signalling
    • H04L1/0026—Transmission of channel quality indication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
    • Y02D30/00—Reducing energy consumption in communication networks
    • Y02D30/50—Reducing energy consumption in communication networks in wire-line communication networks, e.g. low power modes or reduced link rate

Definitions

  • the present invention relates generally to communication, and more specifically to transmission mode and rate selection for a wireless communication system.
  • a wireless multiple-input multiple-output (MIMO) system employs multiple (N T ) transmit antennas at a transmitting entity and multiple (N R ) receive antennas at a receiving entity for data transmission.
  • a MIMO channel formed by the N T transmit antennas and N R receive antennas may be decomposed into Ns spatial channels, where N S ⁇ min ⁇ N T , N R ⁇ .
  • the N S spatial channels may be used to transmit data in parallel to achieve higher throughput and/or redundantly to achieve greater reliability.
  • Each spatial channel may experience various deleterious channel conditions such as, e.g., fading, multipath, and interference effects.
  • the N S spatial channels may experience different channel conditions and may achieve different signal-to-noise-and-interference ratios (SNRs).
  • SNRs signal-to-noise-and-interference ratios
  • the SNR of each spatial channel determines its transmission capacity, which is typically quantified by a particular data rate that may be reliably transmitted on the spatial channel.
  • the channel condition changes over time and the SNR of each spatial channel also changes over time.
  • the different SNRs for different spatial channels plus the time varying nature of the SNR for each spatial channel make it challenging to efficiently transmit data in a MIMO system.
  • a method in which the age of channel information available for use to transmit data via a wireless channel is determined.
  • a transmission mode is selected from among multiple transmission modes based on the age of the channel information.
  • Data is transmitted via the wireless channel in accordance with the selected transmission mode.
  • an apparatus which includes a controller and a spatial processor.
  • the controller determines the age of channel information available for use to transmit data via a wireless channel and selects a transmission mode from among multiple transmission modes based on the age of the channel information.
  • the spatial processor spatially processes data in accordance with the selected transmission mode.
  • an apparatus which includes means for determining the age of channel information available for use to transmit data via a wireless channel and means for selecting a transmission mode from among multiple transmission modes based on the age of the channel information.
  • a method in which channel state information indicative of received signal quality for a wireless channel used for data transmission is obtained.
  • the age of the channel state information is determined.
  • At least one rate is selected for data transmission via the wireless channel based on the channel state information and the age of the channel state information.
  • an apparatus which includes a controller and a data processor.
  • the controller obtains channel state information indicative of received signal quality for a wireless channel used for data transmission, determines the age of the channel state information, and selects at least one rate for data transmission via the wireless channel based on the channel state information and the age of the channel state information.
  • the data processor processes data in accordance with the at least one rate selected by the controller.
  • an apparatus which includes means for obtaining channel state information indicative of received signal quality for a wireless channel used for data transmission, means for determining the age of the channel state information, and means for selecting at least one rate for data transmission via the wireless channel based on the channel state information and the age of the channel state information.
  • a method in which the age of channel information available for use to transmit data via a wireless channel is determined.
  • a transmission mode is selected from among multiple transmission modes based on the age of the channel information.
  • Channel state information indicative of received signal quality for the wireless channel is obtained.
  • the age of the channel state information is determined.
  • At least one rate is selected for data transmission based on the channel state information and the age of the channel state information.
  • Data is transmitted via the wireless channel in accordance with the transmission mode and the at least one rate selected for data transmission.
  • the transmission mode and rate selection techniques described herein may be used for various wireless communication systems. These techniques may be used for single-carrier as well as multi-carrier systems. These techniques may also be used for time division duplex (TDD) as well as frequency division duplex (FDD) systems.
  • TDD time division duplex
  • FDD frequency division duplex
  • the downlink (or forward link) and uplink (or reverse link) are allocated different frequency bands, and the channel response for one link may not correlate well with the channel response for the other link.
  • the downlink and uplink share the same frequency band, and a high degree of correlation normally exists between the downlink and uplink channel responses. Pilot transmission, channel estimation, and spatial processing may be performed in a manner to take advantage of this correlation.
  • station A is a transmitting entity and station B is a receiving entity for a data transmission from station A to station B.
  • Each station may be an access point (which is also referred to as a base station) or a user terminal (which is also referred to as a mobile station, a user equipment, a wireless device, and so on).
  • the exemplary MIMO system supports multiple transmission modes for improved performance and greater flexibility.
  • Each transmission mode may perform spatial processing (if at all) in a different manner and may or may not require channel information for spatial processing.
  • Table 1 lists some exemplary transmission modes and their short descriptions.
  • Table 1 Transmission Mode Description Steered mode Multiple data streams are transmitted on multiple orthogonal spatial channels of a MIMO channel. Unsteered mode Multiple data streams are transmitted on multiple spatial channels of the MIMO channel.
  • the steered mode uses channel information (e.g., eigenvectors) to transmit data on orthogonal spatial channels (or "eigenmodes") of a MIMO channel.
  • channel information e.g., eigenvectors
  • eigenmodes orthogonal spatial channels
  • the unsteered mode does not need any channel information to transmit data on spatial channels of the MIMO channel.
  • the MIMO system may employ spatial spreading for the unsteered mode to enhance performance.
  • station A performs spatial processing with different steering matrices so that a data transmission observes an ensemble of effective channels and is not stuck on a single bad channel realization for an extended period of time. Consequently, performance is not dictated by the worst-case channel condition.
  • Each transmission mode has different capabilities and requirements.
  • the steered mode can typically achieve better performance and may be used if station A has sufficient channel information to transmit data on orthogonal spatial channels.
  • the unsteered mode does not require channel information, but performance may not be as good as the steered mode.
  • a suitable transmission mode may be selected for use depending on the available channel information, the capabilities of stations A and B, system requirements, and so on.
  • N S eigenmodes of the MIMO channel formed by the N T transmit antennas at station A and the N R receive antennas at station B.
  • CCMI channel correlation matrix inversion
  • station B may perform MMSE spatial processing for both the steered and unsteered modes.
  • different matrices H ⁇ eff s and H ⁇ eff u are used for the steered and unsteered modes, respectively.
  • Station B may recover the data symbols in s using CCMI or MMSE receiver spatial processing.
  • the spatial filter matrix would be derived based on H instead of H ⁇ eff u .
  • Station A performs spatial processing with V for the steered mode.
  • Station B performs spatial matched filtering with U (or with H and V ) for the steered mode and with H and V ss for the unsteered mode.
  • An estimate of H may be obtained by one station based on an "unsteered MIMO" pilot sent by the other station.
  • a complete unsteered MIMO pilot may be sent in N (consecutive or non-consecutive) symbol periods, or one symbol period for each chip of the orthogonal sequence.
  • the receiving entity can perform the complementary processing on the received pilot to estimate H . For simplicity, the following description assumes no error in channel estimation.
  • the downlink and uplink channel responses may be assumed to be reciprocal of one another. That is, if H represents a channel response matrix from antenna array X to antenna array Y, then a reciprocal channel implies that the coupling from array Y to array X is given by H T .
  • the responses of the transmit and receive chains at station A are typically different from the responses of the transmit and receive chains at station B. Calibration may be performed to derive correction matrices that can account for the difference in the responses of the transmit and receive chains at the two stations. The application of the correction matrices at these two stations allows a calibrated channel response for one link to be expressed as a transpose of a calibrated channel response for the other link.
  • U and V are matrices of left and right eigenvectors of H ab
  • V * and U * are matrices of left and right eigenvectors of H ba .
  • Stations A and B may use the matrices V and U *, respectively, for spatial processing to transmit data for the steered mode.
  • one station may perform the singular value decomposition to obtain either V or U *.
  • This station may then transmit a "steered MIMO" pilot, which is a pilot sent on the eigenmodes of the MIMO channel.
  • the other station may then estimate its matrix of eigenvectors based on the steered MIMO pilot.
  • Pilot and data may be transmitted in various manners in the MIMO system.
  • station A uses channel information (or "eigensteering" information) to transmit data on the eigenmodes of the MIMO channel.
  • the channel information may be in the form of H (which may be obtained from an unsteered MIMO pilot sent by station B) or in the form of U or V (which may be obtained from a steered MIMO pilot sent by station B).
  • Station B also uses channel information (e.g., H , U , or V for the steered mode, and H for the unsteered mode) to recover a data transmission from station A.
  • station B may estimate the received SNRs for the spatial channels, determine the rate(s) supported by the received SNRs, and send either the received SNRs or the supported rate(s) to station A.
  • Station A may then select a suitable transmission mode and suitable rate(s) for data transmission to station B based on the received feedback and possibly other information.
  • the rates selected by station B are referred to as the initial rates, and the rates selected by station A are referred to as the final rates. Also for clarity, the following description assumes that station B sends rate information (instead of SNR information) back to station A.
  • FIG. 1A shows an exemplary pilot and data transmission scheme 100 for the MIMO system.
  • station A transmits an unsteered MIMO pilot (block 112).
  • Station B receives and processes the unsteered MIMO pilot and obtains an estimate of the channel response matrix H (block 114).
  • Station B also estimates the received SNRs for the (orthogonal or non-orthogonal) spatial channels of the MIMO channel based on the received pilot (block 116).
  • Station B also determines either an initial rate for each eigenmode (for the steered mode) or a single initial rate for the MIMO channel (for the unsteered mode) based on the received SNRs (also block 116).
  • the initial rate(s) are applicable for a data transmission from station A to station B.
  • Station B transmits either an unsteered MIMO pilot or a steered MIMO pilot using the eigenvectors derived from H (block 118).
  • Station A receives and processes the steered or unsteered MIMO pilot to obtain a channel estimate for the link from station A to station B (block 120).
  • Station B also sends the initial rate(s) to station A (block 122).
  • Station A receives the initial rate(s) and determines a transmission mode and final rate(s) to use for data transmission to station B, as described below (block 124).
  • Station A then transmits data to station B using the selected transmission mode and final rate(s) (block 126).
  • Station B receives and processes the data transmission from station A (block 128).
  • FIG. 1B shows another exemplary pilot and data transmission scheme 102 for the MIMO system.
  • station B transmits an unsteered MIMO pilot (block 112).
  • Station A receives and processes the unsteered MIMO pilot and obtains an estimate of the channel response matrix H (block 114).
  • Station A then transmits either an unsteered MIMO pilot or a steered MIMO pilot using the eigenvectors derived from H (block 118).
  • Station B receives and processes the steered or unsteered MIMO pilot to obtain a channel estimate for the link from station A to station B (block 120).
  • the remaining processing for blocks 116, 122, 124, 126 and 128 are as described above for FIG. 1A .
  • pilots may be transmitted in various manners to allow both stations A and B to obtain a channel estimate for the link from station A to station B. Both stations may transmit an unsteered MIMO pilot. Alternatively, one station may transmit an unsteered MIMO pilot, and the other station may transmit a steered MIMO pilot. In this case, either station A or B may transmit the unsteered MIMO pilot, as shown in FIGS. 1A and 1B .
  • FIG. 2 shows an exemplary frame structure 200 that may be used for the MIMO system.
  • Data transmission occurs in units of frames, with each frame spanning a particular time duration (e.g., 2 msec).
  • Each frame may be partitioned into (1) a downlink phase during which data and pilot may be sent on the downlink and (2) an uplink phase during which data and pilot may be sent on the uplink.
  • a MIMO pilot may or may not be sent on the downlink
  • a MIMO pilot may or may not be sent on the uplink.
  • Station B may estimate the received SNRs for the spatial channels based on a steered or unsteered MIMO pilot received from station A.
  • the received SNR is dependent on the spatial processing performed by both stations A and B.
  • the quantity P m n / ⁇ noise 2 is the SNR (in linear units) prior to the receiver spatial processing.
  • the quantities SNR fci,m ( n ), SNR mmse,m ( n ), and SNR ccmi,m ( n ) are the SNRs (in units of decibel (dB)) after the receiver spatial processing and are also referred to as the received SNRs.
  • SNR refers to received SNR unless noted otherwise.
  • the Ns spatial channels achieve similar SNRs because of the spatial spreading with the matrix V ss . Consequently, the same rate may be used for all of the data streams sent on these spatial channels.
  • the SNR averaging may be done in linear unit, as shown in equations (17) and (18), or in dB.
  • station B may determine an initial rate for each eigenmode based on its SNR m ( n ), which may be equal to SNR fcsi, m ( n ) computed as shown in equation (14) or equal to SNR mmse,m ( n ) computed as shown in equation (15).
  • the MIMO system may support a set of rates, and each rate may be associated with a particular data rate, a particular coding scheme, a particular modulation scheme, and a particular minimum SNR required to achieve a specified desired level of performance (e.g., 1% packet error rate).
  • the required SNR for each non-zero rate may be obtained by computer simulation, empirical measurements, and so on.
  • the set of supported rates and their required SNRs may be stored in a look-up table.
  • the SNR m ( n ) for each eigenmode may be provided to the look-up table, which then returns a rate R m ( n ) supported by that SNR.
  • the rate R m ( n ) is associated with the highest data rate and a required SNR that is less than or equal to SNR m ( n ) , or SNR req ( R m ( n )) ⁇ SNR m ( n ) .
  • station B may determine an initial rate for the MIMO channel based on SNR( n ), which may be equal to SNR mmse ( n ) computed as shown in equation (17) or equal to SNR ccmi ( n ) computed as shown in equation (18).
  • SNR( n ) may be provided to a look-up table, which then returns a rate R ( n ) supported by the MIMO channel for the unsteered mode for that SNR.
  • the same or different look-up tables may be used for the steered and unsteered modes.
  • Station B may make an initial determination as to the transmission mode and rate(s) to use for a data transmission from station A to station B.
  • Station A may make a final determination as to the transmission mode and rate(s) to use for this data transmission based on feedback received from station B and other pertinent information.
  • Station A may select which transmission mode to use for data transmission based on the age of the channel information available in the current frame and possibly other information regarding the MIMO channel.
  • the characteristic of the MIMO channel may vary over time due to a number of factors such as, for example, fading, multipath, and interference. For a time variant system, the accuracy/reliability of the channel information degrades over time. Poor performance may be obtained by using inaccurate/unreliable channel information for data transmission.
  • the age of the channel information may be determined based on the age of the MIMO pilot used to derive the channel information. The age of the MIMO pilot may be determined as described below.
  • a MIMO pilot may be transmitted in each frame, or periodically in every few frames, or sporadically.
  • Station A may derive an estimate of H based on an unsteered MIMO pilot received from station B and may decompose H to obtain the matrix V of eigenvectors used to transmit data on the eigenmodes of the MIMO channel.
  • Station A may also obtain the eigenvectors directly from a steered MIMO pilot received from station B. However, this steered MIMO pilot is transmitted by station B using eigenvectors in U , which are derived from an estimate of H obtained by station B from an unsteered MIMO pilot sent by station A.
  • the eigenvectors in V obtained by station A from the steered MIMO pilot sent by station B are, in effect, derived from the unsteered MIMO pilot sent by station A.
  • the quality of the eigenvectors in V derived from the steered MIMO pilot received from station B is thus dependent on (and is only as good as) the quality of the corresponding unsteered MIMO pilot sent by station A, from which H and U are derived.
  • Station A may keep track of when MIMO pilots are transmitted to and received from station B. For example, station A may keep a record of (1) the time each unsteered MIMO pilot is transmitted, (2) the time each steered MIMO pilot is transmitted, (3) the time each unsteered MIMO pilot is received, and (4) the time each steered MIMO pilot is received.
  • This record may be maintained in various formats. For example, the record may contain, for each frame n , four time entries for the four MIMO pilot events. If a MIMO pilot was not transmitted or received in a given frame n , then the time entry for that MIMO pilot for a prior frame n - 1 may be copied and stored for frame n .
  • station A may readily determine (1) the time that the latest (or most recent) unsteered MIMO pilot was transmitted, which is denoted as t tx u A ⁇ B , n , (2) the time the latest steered MIMO pilot was transmitted, which is denoted as t tx s A ⁇ B , n , (3) the time the latest unsteered MIMO pilot was received, which is denoted as t rx u A ⁇ B , n , and (4) the time the latest steered MIMO pilot was received, which is denoted as t rx s A ⁇ B , n . Station A may use this information to determine the age and quality of the channel information that is currently available.
  • Table 2 shows a list of variables used in the description below. Table 2 Symbol Description t tx u A ⁇ B , n The latest time that station A transmits an unsteered MIMO pilot to station B, as determined in frame n . t tx s A ⁇ B , n The latest time that station A transmits a steered MIMO pilot to station B, as determined in frame n . t rx u A ⁇ B , n The time at which station A receives the latest unsteered MIMO pilot from station B, as determined in frame n . t rx s A ⁇ B , n The time at which station A receives the latest steered MIMO pilot from station B, as determined in frame n .
  • Th age rate Maximum age to permit use of the SNR/rate information.
  • SNR( A ⁇ B,n ) Set of SNRs obtained by station A from station B (e.g., derived from initial rate(s) received from station B).
  • t snr ( A ⁇ B,n ) The time at which SNR( A ⁇ B,n ) was obtained by station A.
  • Station A may determine the age of the channel information available in the current frame n (or the "current channel information") as follows. If the current channel information is derived from an unsteered MIMO pilot received from station B, then the age of this information is equal to the age of the unsteered MIMO pilot. However, a delay of d pilot u is incurred to process the unsteered MIMO pilot to obtain the channel information or, equivalently, the channel information is available d pilot u seconds after receiving the unsteered MIMO pilot.
  • the latest unsteered MIMO pilot that could have been used to derive the current channel information was received at least d pilot u seconds earlier and may be identified as follows: max i t current ⁇ t rx u A ⁇ B , i ⁇ d pilot u . If the latest unsteered MIMO pilot for the current frame n was received at least d pilot u seconds earlier, then this unsteered MIMO pilot was used to derive the current channel information. However, if the latest unsteered MIMO pilot for the current frame n was received less than d pilot u seconds earlier, then this unsteered MIMO pilot was not used to derive the current channel information.
  • Equation (19) determines the most recent frame i in which the latest unsteered MIMO pilot for that frame i could have been used to derive the current channel information.
  • the age of this information is equal to the age of the corresponding unsteered MIMO pilot from which the steered MIMO pilot is derived.
  • a delay of d pilot s is incurred by station A to process the steered MIMO pilot received from station B, and a delay of d pilot u is incurred by station B to process the corresponding unsteered MIMO pilot sent by station A.
  • the latest unsteered MIMO pilot that could have been used to derive the current channel information was received at least d pilot s + d pilot u seconds earlier and may be identified as follows: max i t current ⁇ t rx s A ⁇ B , i ⁇ d pilot s AND max j t rx s A ⁇ B , i ⁇ t tx u A ⁇ B , j ⁇ d pilot u . Equation (21) determines the most recent frame j in which the latest unsteered MIMO pilot for that frame j could have been used to derive the current channel information.
  • the transmission mode may also be selected based on other pertinent information. For example, the time variant nature of the MIMO channel may be considered. If the MIMO channel is relatively static (e.g., for fixed stations A and B), then the channel information may be relatively accurate and valid over a longer time period. Conversely, if the MIMO channel changes fairly rapidly (e.g., for mobile stations A and/or B), then the channel information may be accurate over a shorter time period.
  • the time variant nature of the MIMO channel may be accounted for in the computation of the age of the channel information and/or in the age threshold, Th age steer .
  • Age u and Age s may be a function of channel type (e.g., fast or slow fading), different age thresholds may be used for different channel types, and so on.
  • Station A may select the final rate(s) for data transmission to station B based on the age of the underlying MIMO pilot used to derive the initial rate(s).
  • the actual rate(s) supported by the link from stations A to B are dependent on the received SNRs at station B, which may be estimated based on either a steered MIMO pilot or an unsteered MIMO pilot received from station A.
  • the received SNRs may be converted to initial rate(s), which may then be sent back to station A.
  • Station A may estimate the received SNRs at station B based on the initial rate(s) obtained from station B. For example, station A may provide each initial rate to an inverse look-up table, which may then provide the required SNR for the initial rate.
  • the set of SNRs available to station A in the current frame n (or the "current SNR information") is denoted as SNR( A ⁇ B,n ) and is obtained at time t snr ( A ⁇ B,n ) .
  • a delay of d snr is incurred in order (1) for station B to process a steered or unsteered MIMO pilot to estimate the received SNRs, derive the initial rate(s), and send the initial rate(s) back to station A and (2) for station A to process the initial rate(s) to obtain the current SNR information.
  • the latest MIMO pilot that could have been used to obtain the current SNR information was sent by station A at least d snr seconds earlier and may be identified as follows: t snr A ⁇ B , n ⁇ max k t tx u A ⁇ B , k , t tx s A ⁇ B , k ⁇ d snr .
  • Equation (25) determines the most recent frame k in which the latest steered or unsteered MIMO pilot for that frame k could have been used to derive the current SNR information.
  • the final rate(s) may be selected based on the current SNR information, the age of the SNR information, and possibly other information. For example, if the age of the current SNR information exceeds an SNR age threshold (or Age snr_inf n > Th age snr ), then the SNR information may be deemed to be too stale and discarded from use. In this case, the most robust transmission mode and the lowest rate (e.g., the lowest rate in the unsteered mode) may be used for data transmission to station B. If the age of the current SNR information is less than the SNR age threshold, then the SNRs obtained by station A may be adjusted based on the age of the SNR information, and the adjusted SNRs may then be used to select the final rate(s). The SNR adjustment may be performed in various manners.
  • station A may receive an initial rate for each eigenmode m , determine the required SNR for each eigenmode based on the initial rate for that eigenmode, and adjust the required SNR for each eigenmode based on the age of the SNR information.
  • station A may receive an initial rate for each eigenmode and may determine a single final rate for data transmission in the unsteered mode.
  • the number of spatial channels to use for data transmission in the current frame n may be determined by counting the number of "good" eigenmodes with adjusted SNRs greater than an SNR threshold, SNR th . For each eigenmode m , if SNR adj , m u n ⁇ SNR th , then eigenmode m is counted for N sch ( n ). The number of spatial channels to use for the unsteered mode is thus less than or equal to the number of eigenmodes, or N sch ( n ) ⁇ N S .
  • Station B selects the initial rate for each eigenmode based on an assumption that all N s eigenmodes are used for data transmission and that equal transmit power is used for all eigenmodes. If less than Ns spatial channels are used for the unsteered mode, then higher transmit power may be used for each selected spatial channel.
  • the first term on the right hand side in equation (30) accounts for the use of higher transmit power for each spatial channel if less than N s spatial channels are selected for use.
  • the second term on the right hand side in equation (30) is the average SNR (in dB) for the N sch ( n ) spatial channels selected for use in frame n .
  • Station A may provide the average SNR to a look-up table, which then provides the final rate for the unsteered mode.
  • Station A may use the same look-up table that station B used to obtain an initial rate for the unsteered mode, or a different look-up table.
  • station A may receive a single initial rate for the unsteered mode from station B.
  • station A may determine the required SNR for the unsteered mode based on the initial rate, adjust the required SNR based on the age of the SNR information, and determine the final rate for the unsteered mode based on the adjusted SNR.
  • the final rate(s) may also be determined based on other pertinent information such as the time variant nature of the MIMO channel.
  • the SNR back-off, SNR age_bo ( n ), and/or the age threshold, Th age rate may be a function of channel type (e.g., fast or slow fading).
  • the SNR back-off was computed based on a linear function of age, as shown in equation (27). In general, the SNR back-off may be any linear or non-linear function of age and/or other parameters.
  • FIG. 3 shows a flow diagram of a process 300 for selecting a transmission mode for data transmission in a wireless system.
  • channel information used to transmit data via a wireless channel is obtained (block 312).
  • the channel information may comprise eigenvectors used to transmit data on eigenmodes of a MIMO channel and may be obtained from a steered or unsteered MIMO pilot.
  • the age of the channel information is determined (block 314). This may be achieved by determining the age of the (e.g., unsteered MIMO) pilot from which the channel information is derived.
  • a transmission mode is then selected from among multiple supported transmission modes based on the age of the channel information and possibly other information (e.g., the time variant characteristic of the MIMO channel, the capability of the receiving entity, and so on) (block 316). Data is then processed and transmitted via the wireless channel in accordance with the selected transmission mode (block 318).
  • a system may support any transmission mode and any number of transmission modes.
  • a system may support a transmission mode in which data is transmitted on orthogonal spatial channels with spatial spreading, a transmission mode in which data is transmitted on orthogonal spatial channels without spatial spreading (the steered mode), a transmission mode in which data is transmitted on spatial channels with spatial spreading (the unsteered mode), a transmission mode in which data is transmitted on spatial channels without spatial spreading, a transmission mode in which data is transmitted on a single best spatial channel without spatial spreading, a transmission mode in which data is transmitted from a single transmit antenna, and so on, or any combination thereof.
  • FIG. 4 shows a flow diagram of a process 400 for performing rate selection in a wireless system.
  • channel state information indicative of the received signal quality for a wireless channel used for data transmission is obtained (block 412).
  • the channel state information may be in the form of received SNRs, initial rates, and so on, and may be determined by a receiving entity and sent to a transmitting entity.
  • the age of the channel state information is determined (block 414). This may be achieved by determining the age of the (e.g., steered or unsteered MIMO) pilot from which the channel state information is derived.
  • One or more final rates are then selected based on the channel state information, the age of the channel state information, and possibly other information (block 416).
  • the final rate(s) may be determined based on the transmission mode selected for use (e.g., steered or unsteered mode), a back-off factor that is dependent on the age of the channel state information (e.g., SNR age_bo ( n )), a back-off factor that is dependent on the selected transmission mode (e.g., SNR bo s or SNR bo u ) , the time variant characteristic of the wireless channel, and so on.
  • Data is then processed and transmitted via the wireless channel in accordance with the selected final rate(s) (block 418).
  • the techniques described herein select a transmission mode and final rate(s) for data transmission based on the most current information available at the transmitting station A and the age of this information.
  • the channel information used for transmission mode selection and the channel state information used for rate selection may be derived from the same or different MIMO pilots. Different transmission modes and rates may be selected for different frames based on the same information due to the aging of the information and possibly other factors.
  • the transmission mode and rate selection techniques may be used for a multi-carrier MIMO system.
  • Multiple carriers may be provided by orthogonal frequency division multiplexing (OFDM) or some other constructs.
  • OFDM effectively partitions the overall system bandwidth into multiple (N F ) orthogonal subbands, which are also referred to as tones, subcarriers, bins, and frequency channels.
  • N F orthogonal frequency division multiplexing
  • each subband is associated with a respective subcarrier that may be modulated with data.
  • spatial processing may be performed on each of the subbands used for data transmission.
  • a channel response matrix H ( k,i ) may be obtained for each subband k in symbol period i and decomposed to obtain the N S eigenmodes of that subband.
  • a "wideband" eigenmode may be defined as the set of same-order eigenmode of all N F subbands after the ordering.
  • the m -th wideband eigenmode thus includes the m -th eigenmode of all subbands.
  • Each wideband eigenmode is associated with a respective set of N F eigenvectors for the N F subbands.
  • the transmission mode and rate selection may then be performed for the N S wideband eigenmodes, e.g., similar to that described above for a single-carrier MIMO system.
  • FIG. 5 shows a block diagram of transmitting station A 510 and receiving station B 550.
  • a transmit (TX) data processor 520 receives traffic data from a data source 512, processes (e.g., formats, codes, interleaves, and modulates) the traffic data, and provides data symbols.
  • TX transmit
  • For the unsteered mode multiple data streams may be sent on multiple spatial channels, and one final rate may be used for all streams.
  • a TX spatial processor 530 performs spatial processing on the data symbols and pilot symbols for the selected transmission mode and provides N T streams of transmit symbols to N T transmitter units (TMTR) 532a through 532t. Each transmitter unit 532 receives and conditions a respective transmit symbol stream to generate a corresponding modulated signal. N T modulated signals from transmitter units 532a through 532t are transmitted from N T antennas 534a through 534t, respectively.
  • N R antennas 552a through 552r receive the modulated signals transmitted by station A, and each antenna provides a received signal to a respective receiver unit (RCVR) 554.
  • Each receiver unit 554 performs processing complementary to that performed by transmitter units 532 and provides received symbols.
  • a receive (RX) spatial processor 560 performs spatial matched filtering on the received symbols from all N R receiver units 554 based on a spatial filter matrix M ( n ) and provides detected data symbols.
  • the matrix M ( n ) is derived based on the selected transmission mode and the receiver processing technique selected for use (e.g., full-CSI, MMSE, or CCMI).
  • An RX data processor 570 processes (e.g., symbol demaps, deinterleaves, and decodes) the detected data symbols and provides decoded data for station B.
  • Channel estimators 538 and 578 perform channel estimation for stations A and B, respectively.
  • Controllers 540 and 580 control the operation of various processing units at stations A and B, respectively.
  • Memory units 542 and 582 store data and program codes used by controllers 540 and 580, respectively.
  • channel estimator 578 may estimate the channel response for the MIMO channel from station A to station B and the received SNRs for the spatial channels of the MIMO channel. Controller 580 may determine initial rate(s) based on the received SNRs and provide feedback CSI, which may comprise the initial rate(s).
  • the feedback CSI is processed by a TX data processor 590 and further multiplexed with pilot symbols and spatially processed for the steered or unsteered mode by a TX spatial processor 592 to generate N R transmit symbol streams.
  • N R transmitter units 554a through 554r then condition the N R transmit symbol streams to generate N R modulated signals, which are sent via N R antennas 552a through 552r.
  • the modulated signals from station B are received by N T antennas 534 and processed by N T receiver units 532 to obtain received symbols for station B.
  • the received symbols are further processed by an RX spatial processor 544 and an RX data processor 546 to obtain the feedback CSI from station B.
  • Controller 540 receives the feedback CSI, selects the transmission mode and final rate(s) to use for data transmission to station B, provides a rate control to data source 512 and TX data processor 520, and provides the selected transmission mode and channel information (e.g., eigenvectors) to TX spatial processor 530.
  • the transmission mode and rate selection techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof.
  • the processing units used to perform transmission mode and rate selection may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.
  • ASICs application specific integrated circuits
  • DSPs digital signal processors
  • DSPDs digital signal processing devices
  • PLDs programmable logic devices
  • FPGAs field programmable gate arrays
  • processors controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.
  • the transmission mode and rate selection techniques may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein.
  • the software codes may be stored in a memory unit (e.g., memory unit 542 and/or 582 in FIG. 5 ) and executed by a processor (e.g., controller 540 and/or 580).
  • the memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.
  • a method of selecting a transmission mode for data transmission in a wireless communication system comprising determining the age of channel information available for use to transmit data via a wireless channel, and selecting a transmission mode from among a plurality of transmission modes based on the age of the channel information, wherein data is transmitted via the wireless channel in accordance with the selected transmission mode.
  • the method may comprise obtaining eigenvectors used to transmit data on a plurality of orthogonal spatial channels of a multiple-input multiple-output (MIMO) channel, wherein the channel information comprises the eigenvectors.
  • MIMO multiple-input multiple-output
  • the method may comprise determining the age of a pilot used to derive the channel information, and wherein the age of the channel information is determined based on the age of the pilot.
  • the plurality of transmission modes may include a first transmission mode and a second transmission mode, wherein the channel information may be used for spatial processing to transmit data in the first transmission mode, and wherein the channel information may not used for spatial processing to transmit data in the second transmission mode.
  • the selecting one of the plurality of transmission modes may comprise comparing the age of the channel information against a threshold, and selecting the first transmission mode if the age of the channel information is less than or equal to the threshold. Further, the selecting one of the plurality of transmission modes may comprise selecting the second transmission mode if the age of the channel information is greater than the threshold.
  • the method may comprise determining the threshold based on a function of a time variant characteristic of the wireless channel.
  • an apparatus in a wireless communication system comprising a controller to determine the age of channel information available for use to transmit data via a wireless channel and to select a transmission mode from among a plurality of transmission modes based on the age of the channel information, and a spatial processor to spatially process data in accordance with the selected transmission mode. Further, the controller may determine the age of a pilot used to derive the channel information and determines the age of the channel information based on the age of the pilot. Also, the controller may compare the age of the channel information against a threshold and may select a steered mode if the age of the channel information is less than or equal to the threshold, and wherein the spatial processor may use the channel information for spatial processing in the steered mode. Also, the controller may select an unsteered mode if the age of the channel information is greater than the threshold, and wherein the spatial processor may do not use the channel information for spatial processing in the unsteered mode.
  • an apparatus in a wireless communication system comprising means for determining the age of channel information available for use to transmit data via a wireless channel, and means for selecting a transmission mode from among a plurality of transmission modes based on the age of the channel information, wherein data is transmitted via the wireless channel in accordance with the selected transmission mode.
  • the apparatus may comprise means for determining the age of a pilot used to derive the channel information, and wherein the age of the channel information is determined based on the age of the pilot.
  • the means for selecting one of the plurality of transmission modes may comprise means for comparing the age of the channel information against a threshold, and means for selecting a steered mode if the age of the channel information is less than or equal to the threshold, and wherein the channel information is used for spatial processing to transmit data in the steered mode.
  • the means for selecting one of the plurality of transmission modes may comprise means for selecting an unsteered mode if the age of the channel information is greater than the threshold, and wherein the channel information is not used for spatial processing to transmit data in the unsteered mode.
  • a method of performing rate selection for data transmission in a wireless communication system comprising obtaining channel state information indicative of received signal quality for a wireless channel used for data transmission, determining the age of the channel state information, and selecting at least one rate for data transmission via the wireless channel based on the channel state information and the age of the channel state information.
  • a plurality of rates may be supported by the system, and wherein each of the at least one rate is selected from among the plurality of rates supported by the system.
  • the method may comprise determining the age of a pilot used to derive the channel state information, and wherein the age of the channel state information may be determined based on the age of the pilot.
  • the method may comprise determining a back-off factor based on the age of the channel state information, and wherein the at least one rate may be selected based on the back-off factor. Also, the back-off factor may be determined based on a function of time variant characteristic of the wireless channel. Further, the method may comprise determining a back-off factor based on a transmission mode selected for use for data transmission, wherein a plurality of transmission modes may be supported by the system, and wherein the at least one rate is further selected based on the back-off factor. Also, the channel state information may comprise a plurality of initial rates for a plurality of spatial channels of a multiple-input multiple-output (MIMO) channel, one initial rate for each spatial channel.
  • MIMO multiple-input multiple-output
  • the selecting the at least one rate for data transmission may comprise determining a required signal-to-noise-and-interference ratio (SNR) for each of the plurality of spatial channels based on the initial rate for the spatial channel, determining an SNR back-off based on the age of the plurality of initial rates, determining an adjusted SNR for each of the plurality of spatial channels based on the required SNR for the spatial channel and the SNR back-off, and determining a final rate for each of the plurality of spatial channels based on the adjusted SNR for the spatial channel, wherein data is transmitted on the plurality of spatial channels of the MIMO channel using a plurality of final rates determined for the plurality of spatial channels, and wherein the at least one rate selected for data transmission comprises the plurality of final rates.
  • SNR signal-to-noise-and-interference ratio
  • the selecting the at least one rate for data transmission may comprise determining a required signal-to-noise-and-interference ratio (SNR) for each of the plurality of spatial channels based on the initial rate for the spatial channel, determining an SNR back-off based on the age of the plurality of initial rates, determining an adjusted SNR for each of the plurality of spatial channels based on the required SNR for the spatial channel and the SNR back-off, determining an average SNR for a plurality of adjusted SNRs determined for the plurality of spatial channels, and determining a final rate for the plurality of spatial channels based on the average SNR, wherein data is transmitted on the plurality of spatial channels using the final rate determined for the plurality of spatial channels, and wherein the at least one rate selected for data transmission comprises the final rate.
  • the adjusted SNR for each of the plurality of spatial channels may be determined based on a second SNR back-off applicable when one final rate is used for the plurality of spatial channels.
  • an apparatus in a wireless communication system comprising a controller to obtain channel state information indicative of received signal quality for a wireless channel used for data transmission, to determine the age of the channel state information, and to select at least one rate for data transmission via the wireless channel based on the channel state information and the age of the channel state information, wherein a plurality of rates are supported by the system, and wherein each of the at least one rate is selected from among the plurality of rates supported by the system, and a data processor to process data in accordance with the at least one rate selected by the controller.
  • the controller may determine the age of a pilot used to derive the channel state information and determines the age of the channel state information based on the age of the pilot.
  • the controller may determine a back-off factor based on the age of the channel state information and selects the at least one rate further based on the back-off factor. Further, the controller may determine a back-off factor based on a transmission mode selected for use for data transmission and selects the at least one rate further based on the back-off factor.
  • an apparatus in a wireless communication system comprising means for obtaining channel state information indicative of received signal quality for a wireless channel used for data transmission, means for determining the age of the channel state information, and means for selecting at least one rate for data transmission via the wireless channel based on the channel state information and the age of the channel state information, wherein a plurality of rates are supported by the system, and wherein each of the at least one rate is selected from among the plurality of rates supported by the system.
  • the apparatus may comprise means for determining the age of a pilot used to derive the channel state information, and wherein the age of the channel state information is determined based on the age of the pilot.
  • the apparatus may comprise means for determining a back-off factor based on the age of the channel state information, and wherein the at least one rate is further selected based on the back-off factor. Also, the apparatus may comprise means for determining a back-off factor based on a transmission mode selected for use for data transmission, and wherein the at least one rate is further selected based on the back-off factor.
  • a method of performing transmission mode and rate selection in a wireless communication system comprising determining the age of channel information available for use to transmit data via a wireless channel, selecting a transmission mode from among a plurality of transmission modes based on the age of the channel information, obtaining channel state information indicative of received signal quality for the wireless channel, determining the age of the channel state information, and selecting at least one rate for data transmission based on the channel state information and the age of the channel state information, and wherein data is transmitted via the wireless channel in accordance with the transmission mode and the at least one rate selected for data transmission.
  • the apparatus may comprise determining the age of a pilot used to derive the channel information, and wherein the age of the channel information is determined based on the age of the pilot.
  • the apparatus may comprise determining the age of a pilot used to derive the channel state information, and wherein the age of the channel state information is determined based on the age of the pilot.

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US11/101,086 US7564814B2 (en) 2004-05-07 2005-04-06 Transmission mode and rate selection for a wireless communication system
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US8355372B2 (en) 2013-01-15
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US20050249159A1 (en) 2005-11-10
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US7564814B2 (en) 2009-07-21
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